Lubricating oil composition

ABSTRACT

A lubricating oil composition comprising (A) a base oil containing 3% by weight or less of aromatics, 20% by weight or more of monocyclic naphthenes, 50 ppm by weight or less of N and 50 ppm by weight or less of S, having a viscosity (100° C.) of 2 to 50 mm 2  /s, (B) alkyldiphenylamines and/or phenyl-alpha-naphthylamines in an amount of 0.05 to 2% by weight of the total weight of the composition, and (C) C 8  -C 23  MoDTC and/or C 8  -C 18  MoDTP and/or C 8  -C 18  MoDTX in such an amount that the amount of Mo is 50 to 2000 ppm by weight of the total weight of the composition. The lubricating oil composition has high heat resistance, high oxidation stability and excellent lubricating properties, and is particularly useful for a lubricating oil for internal combustion engines and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel lubricating oil composition,and more specifically to a lubricating oil composition having high heatresistance, high oxidation stability and excellent lubricatingproperties, useful as a lubricating oil for internal-combustion engines,automatic transmission gearboxes, dampers, power steering units and thelike, particularly useful as a lubricating oil for internal combustionengines.

2. Description of the Related Art

Lubricating oils have been used for internal combustion engines, and fordriving units and gears such as automatic transmission gearboxes,dampers and power steering unit in order to smoothly operate them. Inparticular, lubricating oils for internal-combustion engines (engineoils) not only lubricate various sliding portions such as a piston ring,a cylinder liner, bearings for a crank shaft and a connecting rod, and avalve-operating mechanism including a cam and a valve lifter, but alsocool the inside of the engines, clean and disperse those products whichare produced by combustion, and prevent the rusting and corrosion of theengines.

Thus, the lubricating oils for internal-combustion engines have beenrequired to have a great variety of properties. Moreover, due to therecent trend toward high-performance, high-output internal-combustionengines and more severe operating conditions, the lubricating oils arerequired to have higher quality. In order to meet this requirement,various additives such as an antiwear agent, a metallic detergent, anonash dispersant and an antioxidant are incorporated into thelubricating oils for internal-combustion engines.

It is particularly important as the essential function of thelubricating oils for internal-combustion engines that the lubricatingoils can ensure the smooth operation of the engines under everycondition to prevent the wear and seizure of the engines. The parts ofthe engines to be lubricated are, in most cases, under the fluidlubrication condition. However, the valve train, and the top and bottomdead centers of a piston tend to be under the boundary lubricationcondition. Antiwear properties under the boundary lubrication conditionare generally imparted by the addition of zinc dithiophosphate (ZnDTP)or zinc dithiocarbamate (ZnDTC).

Energy loss at the friction parts of internal-combustion engines whichare lubricated by lubricating oils is great. For this reason, alubricating oil to which various additives including a friction modifier(FM) are added has been used in order to reduce the friction loss and todecrease the fuel cost (e.g., Japanese Laid-Open Patent Publication No.23595/1991). Lubricating oils for automotive internal-combustion enginesare used at various temperatures, at various revolutions per minute andunder various loads. Therefore, in order to further improve the rate offuel consumption, it is necessary that the lubricating oils be excellentin friction properties under a wide range of conditions under which theyare used.

Besides the above-described properties, high heat resistance, highoxidation stability and moderate viscosity characteristics can bementioned as the properties required for the lubricating oils forinternal-combustion engines.

The present invention is directed to meeting these requirements. Anobject of the present invention is therefore to provide a lubricatingoil composition having excellent lubricating properties, high heatresistance, high oxidation stability and moderate viscositycharacteristics, particularly useful as a lubricating oil forinternal-combustion engines.

DESCRIPTION OF THE FIGURE

FIG. 1 is a diagrammatic view of an apparatus used in the LFW-1 frictiontest in which 1 is the S-test ring, 2 is the R-type block, and 3 is adistortion meter.

SUMMARY OF THE INVENTION

After intensive investigations made for the purpose of developing alubricating oil composition having the above-described advantageousproperties, it has been discovered that the object can be attained by acomposition which is obtainable by adding a predetermined amount of aspecific amine antioxidant, and a predetermined amount of oxymolybdenumsulfide dithiocarbamates (MoDTC), oxymolybdenum sulfideorganophosphorodithioates (MoDTP) or oxymolybdenum sulfidedithioxanthogenates (MoDTX to a lubricating base oil containing a smallamount of aromatics, and having other specific characteristics. Thepresent invention has been accomplished on the basis of the abovefinding.

Namely, the present invention relates to:

(1) a lubricating oil composition characterized by comprising:

(A) a lubricating base oil containing 3% by weight or less of aromatics,20% by weight or more of monocyclic naphthenes, 50 ppm by weight or lessof sulfur and 50 ppm by weight or less of nitrogen, having a viscosityof 2 to 50 mm² /s at 100° C.;

(B) at least one compound selected from diarylamines of the generalformula: ##STR1## wherein R¹, R², R³ and R⁴, which may be the same ordifferent, each represent hydrogen atom or a hydrocarbon group having 3to 18 carbon atoms, provided that at least one of them is thehydrocarbon group, or of the general formula: ##STR2## wherein R⁵ and R⁶are hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms, inan amount of 0.05 to 3% by weight of the total weight of thecomposition; and

(C) at least one compound selected from oxymolybdenum sulfidedithiocarbamates of the general formula: ##STR3## wherein R⁷ and R⁸,which may be the same or different, each represent a hydrocarbon grouphaving 5 to 23 carbon atoms, and m and n are a positive integer,provided that the total number of m and n is 4, oxymolybdenum sulfideorganophosphorodithioates of the general formula: ##STR4## wherein R⁹and R¹⁰, which may be the same or different, each represent ahydrocarbon group having 1 to 18 carbon atoms, and x and y are apositive integer, provided that the total number of x and y is 4, andoxymolybdenum sulfide dithioxanthogenates of the general formula:##STR5## wherein R¹¹ and R¹², which may be the same or different, eachrepresent a hydrocarbon group having 1 to 30 carbon atoms, and X and Y,which may be the same or different, each represent oxygen or sulfuratom, in such an amount that the amount of molybdenum is 50 to 2000 ppmby weight of the total weight of the composition.

Further, preferred embodiments of the present invention are as follows:

(2) the lubricating oil composition as set forth in the above item (1),wherein the lubricating base oil is a hydrogenated oil containing 3% byweight or less of aromatics, 20% by weight or more of monocyclicnaphthenes, and 97% by weight or more of saturated compounds;

(3) the lubricating oil composition as set forth in the item (1) or (2),wherein the lubricating base oil is a hydrogenated oil, the diarylaminesare alkyldiphenylamines containing at least one alkyl group having 3 to18 carbon atoms or phenyl-α-naphthylamines containing an alkyl grouphaving 3 to 18 carbon atoms, and containing the oxymolybdenum sulfidedithiocarbamate;

(4) the lubricating oil composition as set forth in the item (1), (2) or(3), containing the oxymolybdenum sulfide organophosphorodithioates,having an alkyl group having 8 to 18 carbon atoms;

(5) the lubricating oil composition as set forth in the item (1), (2),(3) or (4), containing the oxymolybdenum sulfide dithioxanthogenates,having an alkyl group having 8 to 18 carbon atoms; and

(6) a method for reducing fuel consumption by the use of the lubricatingoil composition described above in internal-combustion engines.

DETAILED DESCRIPTION OF THE INVENTION

In the lubricating oil composition of the present invention, an oilcontaining 3% by weight or less of aromatics, 20% by weight or more ofmonocyclic naphthenes, 50 ppm by weight or less of sulfur and 50 ppm byweight or less of sulfur and 50 ppm by weight or less of nitrogen,having a viscosity of 2 to 50 mm² /s at 100° C. is used as thelubricating base oil, the component (A). The preferable amount of themonocyclic naphthenes is in the range of 25 to 40% by weight. When theamount of the aromatics exceeds 3% by weight, the resulting lubricatingoil composition undergoes deterioration in heat resistance, oxidationstability and lubricating properties. In the case where the amount ofthe monocyclic naphthenes is less than 20% by weight, the resultingcomposition cannot have sufficiently high adaptability to sealingrubber. Further, when the lubricating base oil has a viscosity of lowerthan 2 mm² /s, the resulting composition is poor in the oil-film-formingproperties, and has a shortcoming in that it undergoes a greatevaporation loss. A base oil having a viscosity of higher than 50 mm² /sis also unfavorable because the power loss of the resulting compositioncaused by viscosity resistance is too great. Furthermore, when eithersulfur or nitrogen content exceeds 50 ppm by weight, the oxidationstability and lubricating properties of the resulting composition becomepoor.

Either mineral or synthetic oil can be used as the lubricating base oilas long as it has the aforementioned properties. Specific examples ofthe base oil include raffinates which can be obtained by subjectingstarting materials for lubricating oils derived from naphthene base orparaffin base crude oil by evaporation under normal or reduced pressureto solvent refining, using an aromatic extraction solvent such asphenol, furfural or N-methylpyrrolidone, and hydrogenated oils which canbe obtained by subjecting starting materials for lubricating oils tohydrogenation treatment including hydrocracking reaction. In eitherproduction process, such processes as dewaxing, hydrorefining and claytreatment processes may be optionally adopted in accordance with theconventional manner. Particularly preferable base oils are hydrocrackedoils and wax-isomerized oils.

In the composition of the present invention, at least one compoundselected from diarylamines of the general formula 1!: ##STR6## or of thegeneral formula 2!: ##STR7## is used as the amine oxidant, the component(B).

In the above general formula 1!, R¹, R², R³ and R⁴ each representhydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.Further, although R¹, R², R³ and R⁴ each represent hydrogen atom or ahydrocarbon group having 1 to 18 carbon atoms. Further, although R¹, R²,R³ and R⁴ may be the same or different from one another, it is necessarythat at least one of them be an alkyl group having 3 to 18 carbon atoms.The alkyl group having 3 to 18 carbon atoms may be any of linear,branched and cyclic ones. Examples of such an alkyl group includepropyl, butyl, amyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl andoctadecyl groups of all types, and cyclohexyl, cyclooctyl andcyclododecyl groups.

In the above general formula 2!, R⁵ and R⁶ are hydrogen atom or ahydrocarbon group having 1 to 18 carbon atoms. A preferable hydrocarbongroup is an alkyl group having 3 to 18 carbon atoms, which may be any oflinear, branched and cycilic ones. Examples of such an alkyl groupinclude the same groups as those enumerated in the explanation of R¹,R², R³ and R⁴ in the above general formula 1!. Specifically, thefollowing compounds can be mentioned as the diarylamines:p,p'-dibutyl-diphenylamine, p,p'-dipentyldiphenylamine,p,p'-dihexyl-diphenylamine, p,p'-diheptyldiphenylamine,p,p'-dioctyl-diphenylamine, p,p'-dinonyldiphenylamine,monooctyldiphenyl-amine, monononyldiphenylamine,tetrabutyldiphenylamine, tetrahexyldiphenylamine,tetraoctyldiphenylaniine, tetranonyldiphenylamine, a mixture ofalkyldiphenylamines having 4 to 9 carbon atoms, phenyl-α-naphthylamine,phenyl-β-naphthylamine, butylphenyl-α-naphthylamine,butylphenyl-β-naphthylamine, pentylphenyl-α-naphthylamine,pentylphenyl-β-naphthylamine, hexylphenyl-α-naphthylamine,hexylphenyl-β-naphthylamine, heptylphenyl-α-naphthylamine,heptylphenyl-β-naphthylamine, octylphenyl-α-naphthylamine,octylphenyl-β-naphthylamine, nonylphenyl-α-naphthylamine andnonylphenyl-β-naphthylamine. Particularly preferable diarylamines arep,p'-dioctyldiphenylamine, phenyl-α-naphthylamine andalkylphenyl-α-naphthylamines.

In the composition of the present invention, one or two or more of thealkyldiphenylamines represented by the above general formula 1!, or oneor two or more of the phenyl-α-naphthylamines represented by the abovegeneral formula 2! may be used as the amine oxidant, the component (B).Moreover, one or more of the alkyldiphenylamines represented by thegeneral formula 1!, and one or more of the phenyl-α-naphthylaminesrepresented by the general formula 2! may also be used in combination asthe amine oxidant.

In the present invention, it is necessary to incorporate the amineoxidant, the component (B), into the composition in an amount of 0.05 to3% by weight, preferably 0.2 to 2% by weight of the total weight of thecomposition. When the amount of the amine oxidant is less than 0.05% byweight, the resulting composition cannot have sufficiently highoxidation stability. On the other hand, when the amount is in excess of3% by weight, the effects of the oxidant expected from such an amountcannot be obtained.

In the composition of the present invention, at least one compoundselected from oxymolybdenum sulfide dithiocarbamates (MoDTC) of thegeneral formula 3!: ##STR8## oxymolybdenum sulfideorganophosphorodithioates (MoDTP) of the general formula 4!: ##STR9##and oxymolybdenum sulfide dithioxanthogenates (MoDTX)of the generalformula 5!: ##STR10## is used as the friction modifier; the component(C).

In the above general formula 3!, R⁷ and R⁸ each represent a hydrocarbongroup having 5 to 23 carbon atoms, and they may be the same or differentfrom each other. Examples of the hydrocarbon group having 5 to 23 carbonatoms include a linear or branched alkyl or alkenyl group having 5 to 23carbon atoms, and a cycloalkyl, aryl, alkylaryl or arylalkyl grouphaving 6 to 23 carbon atoms. Preferable hydrocarbon groups are thosehaving 8 to 23 carbon atoms. Specific examples of such hydrocarbongroups include 2-ethylhexyl, n-octyl, nonyl, decyl, lauryl, tridecyl,palmityl, stearyl, oleyl, eicosyl, butylphenyl and nonylphenyl groups.Further, m and n are a positive integer, provided that the total numberof m and n is 4.

In the above general formula 4!, R⁹ and R¹⁰ each represent a hydrocarbongroup having 1 to 18 carbon atoms, and they may be the same or differentfrom each other. Preferable hydrocarbon groups are those having 3 to 18carbon atoms, most preferably 8 to 18 carbon atoms. Examples of thehydrocarbon groups having 3 to 18 carbon atoms include a linear orbranched alkyl or alkenyl group having 3 to 18 carbon atoms, acycloalkyl group having 6 to 18 carbon atoms, an aryl group having 6 to18 carbon atoms, and an alkylaryl or arylalkyl group having 7 to 18carbon atoms. Specific examples of such groups include isopropyl,n-propyl, n-butyl, isobutyl, sec-butyl, amyl, hexyl, cyclohexyl,2-ethylhexyl, n-octyl, nonyl, decyl, lauryl, tridecyl, palmityl,stearyl, oleyl, butylpheyl and nonylphenyl groups. Further, x and y area positive intetger, provided that the total number of x and y is 4.

In the above general formula 5!, R¹¹ and R¹² each represent ahydrocarbon group having 1 to 30 carbon atoms, and they may be the sameor different from each other. Preferable hydrocarbon groups are thosehaving 3 to 20 carbon atoms, most preferably 8 to 18 carbon atoms.Examples of such hydrocarbon groups include a linear or branched alkylor alkenyl group having 5 to 20 carbon atoms, a cycloalkyl group having6 to 20 carbon atoms, and an aryl, alkylaryl or arylalkyl group having 6to 20 carbon atoms. Specific examples of such groups include isopropyl,n-propyl, isobutyl, n-butyl, sec-butyl, amyl, hexyl, cyclohexyl,2-ethylhexyl, n-octyl, nonyl, decyl, lauryl, tridecyl, palmityl,stearyl, oleyl, butylpheyl and nonylphenyl groups. Further, X and Y areoxygen or sulfur atom, and may be the same or different from each other.

In the composition of the present invention, the MoDTC represented bythe above general formula 3! may be used either singly or in combinationof two or more. The MoDTP represented by the general formula 4! may alsobe used either singly or in combination of two or more. Further, theMoDTX represented by the general formula 5! may also be used eithersingly or in combination of two or more.

In the composition of the present invention, it is necessary toincorporate the friction modifier, the component (C), into thecomposition in such an amount that the amount of molybdenum will be 50to 2000 ppm by weight, preferably 100 to 1000 ppm by weight of the totalweight of the composition. When the amount of molybdenum is less than 50ppm by weight, lubricating properties cannot be sufficiently obtained.On the other hand, when the amount of molybdenum is in excess of 200 ppmby weight, lubricating properties expected from such an amount cannot beobtained.

Those additives which have been usually incorporated into theconventional lubricating oils, such as a metallic detergent, a nonashdetergent-dispersant, an antiwear agent, a viscosity index improver, apour point depressant, a rust preventive, a corrosion inhibitor, ananti-foaming agent and other antioxidants, can be added, if necessary,to the lubricating oil composition of the present invention within sucha limit that the object of the present invention can be fully attained.

Examples of the metallic detergent include calcium sulfonate, magnesiumsulfonate, barium sulfonate, calcium phenate, barium phenate, calciumsalicylate and magnesium salicylate. In general, the metallic detergentis incorporated into the composition in an amount of 0.1 to 5% byweight. Examples of the nonash detergent-dispersant include those ofsuccinimide type, succinimide type, benzylamine or its boron derivativetype and ester type. In general, such a detergent is incorporated intothe composition in an amount of 0.5 to 7% by weight.

Examples of the antiwear agent include metallic (Zn, Pb, Sb, Mo, etc.)salts of thiophosphoric acid, metallic (Zn, etc.) salts of thiocarbamicacid, sulfur compounds, phosphoric esters and phosphorous esters. Ingeneral, this agent is incorporated into the composition in an amount of0.05 to 5.0% by weight.

Examples of the viscosity index improver include those ofpolymethacrylate type, polyisobutylene type, ethylene-propylenecopolymer type and styrene-butadiene hydrogenated copolymer type. Ingeneral, such an improver is incorporated into the composition in anamount of 0.5 to 35% by weight.

Examples of the rust preventive include alkenyl succinates and partialesters thereof. Examples of the corrosion inhibitor includebenzotriazole and benzoimidazole. Examples of the anti-foaming agentinclude dimethyl polysiloxane and polyacrylate. These agents may beincorporated into the composition, when necessary.

EXAMPLES

The present invention will now be explained more specifically byreferring to the following Examples. However, the present invention isnot limited by these examples in any way.

The oxidation-induction time and coefficient of friction of thelubricating oil compositions were obtained in the following respectivemanners.

(1) Coefficient of Friction (μ)

LFW-1 friction test was carried out by using an LFW-1 tester shown inFIG. 1 equipped with an R-type block (made of iron) manufactured byFalex Corporation and an S-10 test ring (made of iron) manufactured byFalex Corporation under the following conditions: the number ofrevolutions was 270 rpm, the load was 30 kgf, the temperature of the oilwas 120° C., and the time was 10 minutes. In FIG. 1, reference numeral 1indicates the S-10 test ring, reference numeral 2 indicates the R-typeblock and reference numeral 3 indicates a distortion meter. Load isapplied to the R-type block, and the resistance caused by the rotationof the ring is measured by the distortion meter. The coefficient offriction is calculated from the resistance measured. It is noted thatapproximately half of the ring is immersed in the oil to be tested.

(2) Oxidation-Induction Time (minutes)

The oxidation-induction time was determined by means of differentialthermal analysis which was conducted under oxygen atmosphere, byapplying a load of 20 kgf/cm² and heating the sample to 200° C.

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 7

By the use of a base oil shown in Table 1, a lubricating oil compositionhaving a formulation shown in Table 2 was prepared. The coefficient offriction (μ) and oxidation-induction time (minutes) of the compositionwere obtained. The results are shown in Table 2.

                                      TABLE 1    __________________________________________________________________________                      MONOCYCLIC          VISCOSITY                AROMATIC                      NAPHTHENE                              SULFUR                                    NITROGEN          AT 100° C.                CONTENT                      CONTENT CONTENT                                    CONTENT    BASE OIL          (mm.sup.2 /s)                (wt %)                      (wt %)  (wt. ppm)                                    (wt. ppm)    __________________________________________________________________________     70N  3.1   1.1   32      1.0   0.3    150N-1          5.5   0.5   30      0.5   0.1    350N  9     1.3   32      0.7   0.1    150N-2          5.6   7.0   18      13.0  7.0    150N-3          5.4   2.0   24      137.0 71.0    150N-4          5.7   4.1   19      11.0  89.0    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________                Example 1                     Example 2                          Example 3                               Example 4                                    Example 5                                         Example 6                                              Example 7                                                   Example 8    __________________________________________________________________________    Formulation    Chemical    Composition    (wt %)    Base Oil     70N        balance                     --   --   --   --   --   --   --    150N-1      --   balance                          --   balance                                    balance                                         balance                                              balance                                                   balance    350N        --   --   balance                               --   --   --   --   --    150N-2      --   --   --   --   --   --   --   --    150N-3      --   --   --   --   --   --   --   --    150N-4      --   --   --   --   --   --   --   --    alkyl(C.sub.4-8)diphenylamine                0.5  0.5  0.5  1.5  --   --   0.5  0.5    alkylated(C.sub.8)phenyl-α-                --   --   --   --   0.3  1.2  --   --    naphthylamine    4,4'-methylenebis(2,6-ti-t-                --   --   --   --   --   --   --   --    butylphenol)    MoDTC (C.sub.13)                1.0  1.0  1.0  1.0  1.0  1.0  --   --    MoDTP (C.sub.6)                --   --   --   --   --   --   0.6  --    MoDTX (C.sub.18)                --   --   --   --   --   --   --   0.4    Evaluation    coefficient of friction (μ)                 0.035                      0.039                           0.042                                0.041                                     0.040                                          0.037                                               0.045                                                    0.041    oxidation induction period                24.3 25.6 29.3 33.6 26.6 36.1 25.9 26.5    (minutes)    __________________________________________________________________________

                                      TABLE 3    __________________________________________________________________________    (COMPARATIVE EXAMPLES)                Comp.                     Comp.                          Comp.                               Comp.                                    Comp.                                         Comp.                                              Comp.                Example 1                     Example 2                          Example 3                               Example 4                                    Example 5                                         Example 6                                              Example 7    __________________________________________________________________________    Formulation    Chemical    Composition    (wt %)    Base Oil     70N        --   --   --   --   --   --   --    150N-1      --   --   --   --   --   --   --    350N        --   --   --   --   balance                                         --   --    150N-2      balance                     --   --   balance                                    --   --   balance    150N-3      --   balance                          --   --   --   --   --    150N-4      --   --   balance                               --   --   balance                                              --    alkyl(C.sub.4-8)diphenylamine                0.5  0.5  --   0.5  --   --   0.5    alkylated(C.sub.8)phenyl-α-                --   --   1.2  --   --   --   --    naphthylamine    4,4'-methylenebis(2,6-ti-t-                --   --   --   --   0.5  0.5  --    butylphenol)    MoDTC (C.sub.13)                1.0  1.0  1.0  --   1.0  1.0  --    MoDTP (C.sub.6)                --   --   --   0.6  --   --   --    MoDTX (C.sub.18)                --   --   --   --   --   --   0.4    Evaluation    coefficient of friction (μ)                 0.035                      0.049                           0.051                                0.053                                     0.050                                          0.056                                               0.059    oxidation induction period                15.2 14.3 21.8 13.9 21.3 16.4 15.5    (minutes)    __________________________________________________________________________

All of the lubricating oils of Examples 1 to 8, which are thecompositions of the present invention, have a low coefficient offriction and a long oxidation-induction time. In contrast, thelubricating oil of Comparative Example 1 has a low coefficient offiction but has a short oxidation-induction time. The lubricating oilsof Comparative Examples 2 to 7 are remarkably inferior to those ofExamples of the present invention in both the coefficient of frictionand the oxidation-induction time.

The lubricating oil compositions of the present invention have high heatresistance, high oxidation stability and excellent lubricatingproperties, and are particularly useful for lubricating oils forinternal-combustion engines and the like.

We claim:
 1. A lubricating oil composition comprising:(A) a lubricatingbase oil containing 3% by weight or less of aromatics, 20% by weight ormore of monocyclic naphthenes, 50 ppm by weight or less of sulfur and 50ppm by weight or less of nitrogen, having a viscosity of 2 to 50 mm² /sat 100° C.; (B) at least one compound selected from diarylamines of thegeneral formula: ##STR11## wherein R¹, R², R³, and R⁴, which may be thesame or different, each represent hydrogen atom or a hydrocarbon grouphaving 1 to 18 carbon atoms, provided that at least one of them is ahydrocarbon group having 3 to 18 carbon atoms, or ##STR12## wherein R⁵and R⁶ are hydrogen atom or a hydrocarbon group having 1 to 18 carbonatoms, in an amount of 0.05 to 3% by weight of the total weight of thecomposition, and (C) at least one oxymolybdenum compound selected fromoxymolybdenum sulfide dithioxanthogenates of the general formula:##STR13## wherein R¹¹ and R¹², which may be the same or different, eachrepresent a hydrocarbon group having 1 to 30 carbon atoms, and X and Y,which may be the same or different, each represent oxygen or sulfuratom, in such an amount that the amount of molybdenum is 50 to 2000 ppmby weight of the total weight of the composition.
 2. The lubricating oilcomposition of claim 1 wherein the base oil is a hydrogenated oilcontaining 3% by weight or less of aromatics, 20% by weight of more ofmonocyclic naphthenes and 97% by weight or more of saturated compounds.3. The lubricating oil composition of claim 1 or 2 wherein R¹¹ and R¹²each represent a hydrocarbon group having 8 to 18 carbon atoms.